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2.4
Bacteria
In section 2.1, you learned that bacteria can be classified into two kingdoms:
Archaebacteria and Eubacteria. Before we examine the differences, consider
the similarities all bacteria possess:
• All are unicellular; some stick together in colonies.
• Cells are prokaryotic. Bacteria usually lack membrane-bound internal
structures, having no organized nucleus, vacuoles, mitochondria, or
chloroplasts.
• Cells usually have a single chromosome in the form of a DNA loop.
• Cells reproduce asexually by binary fission.
• Cells thrive only in moist environments and become inactive if the
environment dries up.
Figure 1
(a) Eukaryotic animal cell
(b) Prokaryotic bacterial cell
eukaryotic a type of cell that has a
true nucleus, surrounded by a
nuclear envelope
prokaryotic a type of cell that does
not have its chromosomes
surrounded by a nuclear envelope
plasmid a small piece of genetic
material
(a)
There are two types of cells, eukaryotic and prokaryotic. Cells are placed
into one of these two categories based on how complex their structure and cell
division process is. Plant and animal cells are eukaryotic, meaning “true
nucleus”; their structures were outlined in Unit 1 (see Figure 1 for review).
Bacterial cells are prokaryotic, meaning “before nucleus.” Genetic material
floats freely inside the cell in the form of either a loop or a small piece of
genetic material called a plasmid. Bacteria have a rigid outer wall that gives
them shape. Underneath this wall is a more fluid membrane called the cell or
plasma membrane. There are few recognizable organelles in the cytoplasm,
even under the high magnification of an electron microscope. Table 1
compares some properties of eukaryotic and prokaryotic cells.
Table 1
DID YOU
KNOW
?
The Missing Link
Some scientists believe that
archaebacteria, isolated in the 1970s,
are the link between eubacteria and
eukaryotes. Over 50% of the genes
identified in this kingdom were
previously undiscovered in other
organisms.
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Unit 2
(b)
Properties of Eukaryotic and Prokaryotic Cells
Property
Eukaryotes
Prokaryotes
true nucleus
present
absent
DNA
usually many chromosomes
usually single chromosome
cell division
mitosis
binary fission
ribosomes
larger
smaller
mitochondria
present
absent
chloroplasts
can be present
absent
flagella
complex flagella
simple flagella
size
usually > 2 µm diameter
usually < 2 µm diameter
microorganism
examples
Euglena, Paramecium, Amoeba
Escherichia coli, Bacillus
anthracis
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Section 2.4
Archaebacteria
Archaebacteria (archae means “early” or “primitive”) are derived from one of
the oldest groups of living organisms, perhaps the first on Earth. These
organisms thrive under extreme conditions that other organisms could not
tolerate. Many live without oxygen. Three major groups of archaebacteria
include the thermophiles, the methanogens, and the halophiles (Figure 2).
The thermophiles live in extremely hot environments. Some obtain energy
by oxidizing sulfur, and thrive in and around hot sulfur springs. The
methanogens grow on carbon dioxide and hydrogen gas to produce methane.
They exist in environments such as volcanic deep-sea vents and the intestines
of mammals, including humans. The halophiles live in extremely saline
environments, such as salt flats and evaporation ponds, and are responsible
for the purplish-red colour in these areas. The bright red pigment protects the
cells from intense solar radiation, yet allows them to use sunlight for energy.
Scientists are putting this kingdom to work to benefit society. Methanogens
help digest sewage and oil spills, producing methane, which can be used as an
alternative fuel source. Halophiles are used in cancer research. Enzymes
produced by archaebacteria are used in food processing, perfume
manufacture, and pharmaceuticals. The most famous archaebacterial enzyme
is Taq polymerase, a substance isolated from Thermus aquaticus and used in
molecular biology.
(a) Thermophiles live at temperatures
over 45°C.
(b) Methanogens are found in
environments without oxygen, such as
swamps and marshes.
Figure 2
Examples of archaebacteria
(c) Halophiles colour the landscape
around salt lakes.
Eubacteria
The best representative organism of the kingdom Eubacteria, and likely one of
the most studied organisms in the world, is Escherichia coli (Figure 3). As you
will discover throughout this unit, this organism is both helpful and harmful
to humans. Billions of E. coli live in the human intestine, helping with food
digestion and the synthesis of vitamin K and B complex vitamins. Human and
animal feces also contain billions of E. coli bacteria. High counts of such
bacteria (coliform bacteria) in water indicate fecal contamination and danger
to human health.
Most bacteria range in size from 0.4 µm in diameter to several micrometres
in length. As E. coli is about 1 µm in length, a line of about 250 organisms
could just be seen by the naked eye. The structure of E. coli is illustrated in
Figure 4, on the next page.
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Figure 3
E. coli micrograph
Microbiology 109
pili (singular: pilus): These hairlike
structures help bacteria attach to
each other and to surfaces. They can
also be involved in movement.
genetic material: DNA floats in the
cytoplasm; there is no nucleus. It
usually consists of one large
chromosome ring, and several
smaller ones called plasmids.
cytoplasm: Not divided
into compartments
shape: Rodlike
capsule:
A sticky coating
surrounds some
disease-causing
bacteria, helping
protect them
from destruction
by the host’s
immune system.
ribosomes: Float freely
in the cytoplasm,
making protein
cell membrane:
Regulates movement
of materials in and
out of the cell
flagellum: Flagella
rotate like propellers to
drive the cell through
its aqueous habitat.
Figure 4
E. coli—a typical bacterial cell
Figure 5
Staphylococcus epidermis is
Gram-positive.
cell wall: The cell wall
is Gram-negative and
consists of a thin
peptidoglycan layer, an
outer membrane, and a
periplasm—the area
between the cytoplasmic
membrane and the outer
membrane.
The composition of the cell wall varies among bacteria species and is an
important means of identifying and classifying bacteria. Eubacteria contain a
polymer called peptidoglycan in their cell walls. The Gram staining method
(developed by Danish bacteriologist Hans Christian Gram in 1884) differentiates
between two major cell-wall types. In this method, the bacterial sample is
smeared on a microscope slide, and stained with crystal violet dye (purple).
The dye is then fixed to cells with Gram’s iodine, decolourized with ethanol,
and counterstained with safranin (red).
Gram-positive bacteria such as Staphylococcus (Figure 5) retain the first dye,
appearing purple in the light microscope. In Gram-negative bacteria, the
decolourizer washes out the violet dye, the counterstain is taken up, and the
cells appear pinkish-red. E. coli is a Gram-negative bacterium (Figure 6).
Differences in the amount of peptidoglycan in the wall determine whether
or not the crystal violet is washed out. Gram-positive bacterial walls contain
more peptidoglycan, so these walls retain the purple dye.
Figure 6
E. coli is Gram-negative.
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Unit 2
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Section 2.4
monococcus (1)
diplococcus (2)
bacilli
(found singly or
in pairs or chains)
spirilla
streptococcus (chain)
staphylococcus (clump)
Aside from cell-wall composition, eubacteria can be classified according to
shape, configuration, respiration, and type of nutrition.
Most organisms display one of three basic shapes—spherical, rod-shaped,
or spiral (Figure 7).
After division, many bacteria stay together in groups or clusters rather than
remain as individual cells. Cocci (singular: coccus), bacilli (singular: bacillus),
and sometimes spirilla (singular: spirillum), form pairs, cluster colonies, or
chains (filaments) of cells. For example, Streptococcus mutans, the main cause
of tooth decay, forms chains. Staphylococcus aureus, a common bacterium
found on the skin, forms clumps. When large numbers of cells have grown,
they become colonies. Myxobacteria form specialized colonies in one point of
their growth called fruiting bodies (Figure 8).
Some bacteria are aerobic organisms and must have oxygen to survive.
Bacteria that cause tuberculosis are aerobic organisms. Other bacteria are
anaerobic and can only grow in the absence of oxygen. Organisms causing
gangrene, tetanus, and botulism are anaerobic organisms. Many bacteria can
survive and grow with or without oxygen. E. coli, the bacterium in Figure 4, is
in this category.
Classifying eubacteria by type of nutrition shows the diversity of this
kingdom. Some bacteria are autotrophs, making the food they require from
inorganic substances. Photosynthetic bacteria convert carbon dioxide and
water into carbohydrates by using energy from sunlight. Chemosynthetic
bacteria use chemical reactions rather than sunlight as their energy source.
Most bacteria are heterotrophs, obtaining their nutrients from other
organisms (e.g., by feeding on dead or decaying matter, or as parasites causing
disease by feeding on living tissue).
Reproduction and Growth of Eubacteria
and Archaebacteria
Bacteria reproduce asexually by binary fission. Although it bears some
resemblance to mitosis, binary fission is much simpler. The single strand of
DNA replicates, resulting in identical genetic material being transferred to
each new cell. Following replication of the genetic material, the bacterium
produces a cross wall, dividing the cell into two identical bacteria, which may
separate or remain attached (Figure 9(a), on the next page).
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Figure 7
Basic shapes of bacteria
colony a visible growth of
microorganisms containing
thousands of cells
aerobic requiring oxygen for
respiration
anaerobic conducting respiration
processes in the absence of oxygen
Figure 8
Millions of unicellular myxobacteria
form a fruiting body. In this
configuration, they can last through
harsh conditions for long periods of
time.
CAREER CONNECTION
Bacteriology and microbiology
technologists work in laboratories
culturing bacteria to determine their
identity. Many Canadian colleges
and institutes of technology offer
programs in these fields.
Microbiology 111
endospore a dormant cell of bacilli
bacteria that contains genetic
material encapsulated by a thick,
resistant cell wall. This form of cell
develops when environmental
conditions become unfavourable.
(a)
Sexual reproduction is not common in bacteria. However, conjugation does
occur among some bacteria, such as E. coli and Salmonella. In conjugation
(Figure 9(b)), donor and recipient bacteria make cell-to-cell contact by means
of a special structure called a sex pilus, where plasmids are transferred, giving
the recipient an altered set of characteristics. Following the transfer, the two
bacteria separate. Plasmids can also be transferred by other means.
During unfavourable environmental conditions, some bacteria survive by
forming dormant or resting cells, called endospores (Figure 10). Endospores
are resistant to heat and other extreme conditions, and cannot be easily
destroyed. When suitable growing conditions return, the endospore sprouts or
germinates, and an active bacterium emerges.
attachment
DNA
(b)
DNA replicates
Cytoplasm is divided in two, and
two new cells are formed. In this
case, they also separate.
Figure 9
(a) Binary Fission. Features
normally associated with
mitotic cell division such as
centrioles, spindle fibres, and
visible chromosomes are not
involved in this process.
(b) Conjugation. A sex pilus joins
two bacteria cells, and
plasmids are exchanged.
endospore
Plasma membrane and
cell wall grow.
plasmids
sex pilus
Plasma membrane and wall
material start growing through
the midsection.
Section 2.4 Questions
Understanding Concepts
1. Explain the statement “archaebacteria thrive on extremes.”
2. List six ways in which archaebacteria contribute to human society.
3. Design a graphic organizer that could be used to classify eubacteria
according to shape, respiration, and nutrition. Give examples of each.
4. Where would you expect to find anaerobic bacteria in nature?
5. Why is conjugation considered a form of sexual reproduction?
6. How has endospore formation guaranteed the survival of bacteria?
7. Identify the following bacteria types:
(a)
(b)
(c)
8. In a laboratory experiment, a student grew a colony of bacteria from a fecal
sample. She suspected that the organisms were E. coli. Provide a description
of the anatomy and physiology of E. coli to help with the identification.
Figure 10
An electron micrograph showing an
endospore within a bacterium. A
thickened cell wall forms around the
genetic material and cytoplasm. The
remainder of the original cell
eventually disintegrates.
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Unit 2
Making Connections
9. One of the basic techniques in a college microbiology laboratory is Gram
staining. Research this practice. Outline the steps in the procedure and list
common examples of Gram-positive and -negative bacteria.
GO
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